Field of the Invention
The invention relates to a magnetic resonance apparatus, including a magnet system for generating a steady magnetic field, a coil system for generating a gradient field, an RF transmitter coil, and an RF receiver coil system for detecting magnetic resonance signals generated in an object, which RF receiver coil system has two receiver coils which could be coupled to one another by mutual inductance, a four terminal decoupling network connected to the ends of the two receiver coils being provided to compensate for the mutual inductance, which network contains a first circuit of variable impedance which is connected between first ends of the two receiver coils and a second circuit of variable impedance which is connected between second coils of the two receiver coils. The invention also relates to an RF receiver coil system suitable for use in such an apparatus.
An example of a magnetic resonance apparatus of this kind is known from U.S. Pat. No. 4,769,605. In the known device, the first and the second circuit can each be formed by a variable capacitor. The decoupling network is then very simple and inexpensive, but also has a number of practical drawbacks. A first drawback, mentioned in U.S. Pat. No. 4,769,605, consists in that first and second variable capacitors comprising the first and second circuit, respectively, should have a very small value, for example less than 1 pF, and that it is particularly difficult to accurately manufacture variable capacitors of such a low value. Moreover, these two capacitors should be simultaneously varied, thus increasing the difficulties. Instead of the simple decoupling network comprising only two capacitors, therefore, preferably a much more complex network comprising at least five capacitors is used. A further complication arises because the polarity of any coupling due to the mutual inductance between the receiver coils often is not known in advance. Therefore, the decoupling network must be suitable to compensate for positive as well as negative mutual inductance. The cited document states two methods to achieve this object. In accordance with the first method, the decoupling network is connected to the coils via switches, so that the connections of the network to the coils can be interchanged in the event of a different polarity of the mutual inductance. In accordance with the second method, a very complex decoupling network consisting of at least ten capacitors is used. It will be evident that both methods lead to complex, expensive and vulnerable decoupling networks.